The present invention relates to active geophysical exploration. It has long been known that there are substantial electromagnetic fields associated with the Earth. The origin of these electromagnetic fields is unclear: one theory holds that low-frequency electromagnetic fields are emitted from beneath the surface of the Earth and radiate outward, such that they can be measured by low frequency methods at the surface. Others postulate that currents are generated by oxidation-reduction type reactions taking place where water and hydrocarbons are present, and that the electromagnetic radiation is caused by interaction of these steady and unsteady currents with the Earth's magnetic field. Still others postulate that the radiation is reflected from outside the Earth's atmosphere.
Irrespective of the source, it is well-documented scientific theory that discontinuities in the subterranean structure of the Earth crust cause reflection and refraction of electromagnetic radiation at interfaces between electrically differing materials. Additionally, the distance a transverse electromagnetic wave travels in a material before being substantially absorbed, is a known function of the frequency of the wave (so-called “skin-depth” expression). Thus, it has been hypothesized that prospecting for hydrocarbons, such as oil, gas and coal, as well as precious metals, could be achieved by mapping the strength of electromagnetic waves at various frequencies which naturally emanate from the Earth providing a passive method of subterranean exploration and prospecting techniques.
Utilizing naturally occurring signals seems plausible, but has proven problematic. Because the emitted wave is so random and its exact source not known, obtaining meaningful signal is an extremely difficult task, if not impossible. In addition, since the source and strength is not known, calibration of instruments is a guess. One almost insurmountable problem with these signals is “noise.” That is, there is an extremely low signal-to-noise ratio associated with these low frequency signals, and this high level of noise typically causes interference in detecting those signals that are determinative of geologic formations. For example, even the cycling of the measurement equipment, such as cooling fans, disrupts the signal.
None-the-less, many different passive methods for picking up and determining low frequency electromagnetic waves emanating from the Earth have been proposed. By utilizing an antenna to pick up these naturally occurring frequencies emanating from the Earth's surface, theoretically one can filter, amplify, modify and otherwise process these signals to turn them into a readable signal. Various low pass and high pass filtering techniques have been proposed, and in some cases employed, after the initial amplification of the signal to improve the quality of the signal. This amplification and filtering is known as “conditioning” and/or “pre-conditioning” of the signal and is generally considered to be the most common technique for identifying naturally occurring electromagnetic signals emanating from the Earth.
Many attempts to electronically solve these problems have met with only limited success. For example, U.S. Pat. No. 6,414,492 discloses a system for passively determining physical characteristics of subterranean geological formation that includes an antenna for acquiring low frequency signals naturally emanating from the Earth which signals are first put through a low pass filter and buffer and then converted from analog to digital, stored in a memory buffer, converted to a frequency spectrum by a Fourier transform, and then further processed to display geophysical information versus the depth of such discontinuities.
Unfortunately, irrespective of the methods employed to improve the quality of the signals from passive systems, problems remain with the signal. Signal-to-noise ratios, as well as the randomness of the electromagnetic wave source remain problematic and impede, if not totally prohibit reproducibility. In order to overcome these problems, active methods have been suggested. Well known active methods for determining geologic subterranean surfaces involve seismic methods wherein pre-programmed charges are detonated, sending a mechanical wave through the area to be mapped. These seismic systems do not employ electromagnetic energy. The “mechanical” seismic waves are received with sensitive seismic meters to locate and identify subterranean geologic formations. This methodology employs the concept that discontinuities in subterranean structure reflect mechanical waves, and that different wave frequencies propagate differently in the Earth. The distinguishing features of the seismic method (as with all active methods) is that the original probing signal is generated, and therefore of a known intensity and characterization.
The seismic refraction method is based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers of different velocity. Seismic energy is provided by a source (“shot”) located on the surface. Energy radiates out from the shot point, either traveling directly through the upper layer (direct arrivals), or traveling down to and then laterally along higher velocity layers (refracted arrivals) before returning to the surface. This energy is detected on the surface using a linear array of geophones. Observation of the travel-times of the refracted signals provides information on the depth profile of the refractor.
Use of mechanical wave propagation in a solid media, however, leads to a whole different set of problems. The problem with seismic waves is their propagation patterns within the media-Earth. Being mechanical waves, they have two propagation modes—transversal and compressional. These modes can mix at the boundaries between media resulting in a shear wave making the interpretation of the resulting data extremely difficult. Electromagnetic waves do not have these two propagation modes and, therefore, the signal is “cleaner”. The meaningfulness of the detected seismic signal is reduced by seismic noise as the sound waves bounce around inside the Earth, some canceling and other amplifying.
It would, therefore, be advantageous to have an active electromagnetic wave system that could produce reliable, reproducible data. However, prior art systems have not, to-date, been effective in accomplishing this for a myriad of reasons. One set of methods as illustrated by U.S. Pat. No. 3,636,435 falls into the, so-called, “Induced Polarization” (IP) class of prospecting methods. In accordance with this method, a large transmitting “loop” conductor is laid directly on the ground and fixed in this position while data is being taken. An alternating current is induced in the loop to generate an electromagnetic field. The inductive fields generated induce eddy currents in the Earth and these eddy currents create secondary fields that radiate in all directions to interact with all profiled obstacles, as well as the Ionosphere, i.e. the field radiates up, down and all around. A detecting loop (or pair of loops, etc.) is moved around and/or multiple detecting loops are positioned and held fixed. In either case, changes in the inductive field intensity as a function of position (so-called “gradients”) are recorded. The “Polarization” in IP refers to spatial separation of charged particles in the environment, and not to the vector direction of the electromagnetic fields.
The inherent problems with the IP-class methods are numerous. As the generating loop is near the ground, it is well known that the interaction with the ground is primarily “magnetic induction” and not electromagnetic waves, creating a near field effect. The near field effect is characterized as a region of space close enough to a transmitting antenna so that the strength of the induction field is larger than strength of the radiation field. As is well known, the radiation fields are those electric and magnetic fields that decay as 1/r (where r is the distance from the geodetic center of the antenna) and that collectively carry energy away from the source; while, induction fields are those electric and magnetic fields that decay as 1/r2 and that collectively carry no energy away from the source.
Also, the art generally requires the generating loop, the Earth, and the receiving loop (or loops) all to reach steady-state, meaning that a relatively long time elapses (˜10 seconds) before data is taken. This transmission/reception time allows the generated energy to interact with unintended and interfering objects, including the Ionosphere. The recorded data from these unintended objects is meaningless for the intended purpose and creates undistinguishable interfering signals, i.e. it is generated noise. First, as the Ionosphere is randomly and chaotically changing, this interaction “scrambles” the reflected data. Second, no precise orientation or shape of the generating or receiving loops is maintained. If the field strengths at multiple IP ground receiving loops are differenced, the resulting measurements are basically identical to those obtained via the so-called resistivity sounding prospecting methods.
Another set of active electromagnetic prospecting methods, as illustrated by U.S. Pat. No. 3,500,175, or U.S. Pat. No. 3,617,866, are referred to as Radio Frequency Surveys (RFS's). In these methods, a continuous-wave transmitter (either one actively constructed as part of the apparatus or a “convenient” radio frequency transmitter known to already be in the area) is activated. The transmitter “lights up” the Earth-Ionosphere waveguide. A receiving antenna is moved across the surface region of interest (by human, motor vehicle, or airplane) and the resulting field intensities are recorded as a function of position.
In some methods in this class, where the transmitter is under the control of the person versed in the art, the relative phase between transmitted and received field is also recorded. The data set of interest is the change in received information as a function of changes in the position of the receiving antenna. The precise orientation of the receiver and transmitter is not controlled (except to aid in gathering time-delay (phase) information). Practitioners of RFS typically claim the readings produced indicate something about subsurface conductivities and the method is disclosed and practiced as being at a single, convenient frequency. Again, the interference and uncertainty of the random reflective, verses the emitted signal without time delay, makes the gathered information suspect and impossible of accurate interpretation.
A third broad class of electromagnetic prospecting methods falls into the Ground Penetrating Radar (GPR) class of methods as illustrated by U.S. Pat. No. 5,339,080. As practiced and disclosed, GPR is typically a pulsed method, at a single frequency eliminating ionospheric interference. However, in order to eliminate interaction between the ground and the inductive fields, GPR operates in the meter to centimeter wave region of the electromagnetic spectrum (approximately 100 MHz to 500 MHz). At these wavelengths, the transmitter can be only a few feet above the ground, yet the ground can still be in the far-field. Unfortunately, at these wavelengths, GPR can only “see” into the ground about 10-30 meters at the most (penetration, or skin-depth is a function of frequency). Moreover, GPR, like virtually all radar systems, makes only a field intensity measurement and generally operates at normal incidence (angle of incidence is 90°) to the Earth surface; therefore, no information on the polarization of the fields is measured or can be meaningful. Most GPRs are designed to transmit and receive only at a single fixed frequency.
In both the IP class and the RFS class of methods, there has been recent attention paid to “pulsed” transmission as illustrated in U.S. Pat. Nos. 5,498,958 or 5,796,253. In exemplary disclosures of pulsed methods to-date, the pulsed nature of the transmission is to facilitate attempting to view the “decay” characteristics of the received field intensity. In accordance with this practice, purportedly, the decay characteristics contain intelligible information about the location of buried conductors. Again, the pulsing is not carried out in a way that will limit interaction with the Ionosphere which randomly scrambles the received signal and is interfering (noise.) None of these methods allow the reconstruction of strata thicknesses and composition to produce a three-dimensional display (topology) of the subterranean landscape.
It will be realized that the foregoing discussion and examples of the related art and the scope of the illustrations related thereto are set forth as background only. Their intent is to be exemplary and illustrative of problems related to the art, as well as prior attempts to address these problems, at least in part. They are not, nor are they intended to be exclusive or exhaustive. Nor are they intended, in any manner, to be read as a limitation of the instant disclosure or the appended claims.